Hydraulic systems are known for converting fluid energy, for example, fluid pressure, into mechanical energy. Fluid power may be transferred from a hydraulic pump through fluid conduits to one or more hydraulic actuators. Hydraulic actuators may include hydraulic motors that convert fluid power into shaft rotational power, hydraulic cylinders that convert fluid power into translational motion, or the like.
Some hydraulic systems may include a hydrostatic closed-loop, having a pump coupled to a hydraulic motor through a motor supply conduit and a pump return conduit, such that all of the hydraulic fluid is not returned to a reservoir upon each pass through the closed-loop. But even in so-called closed-loop systems, fluid may leak out of the closed-loop during operation, or be deliberately flushed from the closed-loop to a reservoir to control a hydraulic fluid property such as temperature, viscosity, cleanliness, or the like. Thus, operation of hydrostatic closed-loop systems may benefit from replacement of lost hydraulic fluid during operation.
Hydraulic boost circuits have been proposed to supply makeup hydraulic fluid flow or boost flow to a hydrostatic closed-loop circuit. Some boost circuits employ a single fixed displacement pump coupled to a hydrostatic closed-loop hydraulic circuit to make up lost fluid. However, such systems may lack flexibility in tailoring pump output to boost needs over an operating range. As a result, single fixed displacement boost pumps may provide inadequate flow in some circumstances and excess flow in other circumstances.
Alternatively, boost circuits including a variable displacement boost pump have been proposed to vary the amount of hydraulic fluid makeup flow to the closed-loop at any given time by varying pump displacement. However, variable displacement boost pumps may be prohibitively expensive, large in size, or both.
Japanese Patent Publication 2004-204985 (JP '985), entitled “Flow Rate Backup Circuit to HST Circuit,” purports to address the problem of fixed displacement hydraulic pump flow utilization. The JP '985 publication describes a first fixed displacement pump that supplies flow to a hydrostatic transmission (HST), and a second fixed displacement pump that supplies flow to a work machine actuator or to the HST depending on vehicle rolling speed. However, the JP '985 publication still may not enable tailoring of boost pressure at a confluence of multiple fixed displacement boost pumps.
Accordingly, there is a need for improved apparatus and methods for controlling hydraulic boost circuits at low cost and within space requirements.